Impacts of Persistent Low Cloud on Air Pollution Concentrations during Wintertime Stagnation Conditions

The Yakima Air Wintertime Nitrate Study (YAWNS) took place in January 2013 to investigate the drivers of elevated levels of fine particulate matter (PM2.5) observed in the region during winter stagnation periods. An extensive suite of instruments was deployed to characterize the meteorological, trace gas, and particulate properties. During the study an extended stagnation period occurred. For the first two days of the event, clear sky was present and the air pollution characteristics were consistent with the presence of low-level night time temperature inversions. On the third day, a low-level cloud layer formed that persisted over the valley for the next several days. Coincident with the formation of the cloud layer, the levels of most pollutants dropped to near the atmospheric background and remained low for as long as the cloud layer was present. All measured primary pollutants were affected, including CO2, CO, NOx, VOCs, and black carbon. In contrast, levels of secondary pollutants, most notably aerosol nitrate, did not exhibit the same decline. We argue that the observed behavior in the tracked air pollutants results from persistent atmospheric mixing from the surface to the top of the cloud layer and ventilation through the top of the cloud layer. If such occurrences are common, there may be significant implications for air quality management decision-making.